Hybrid assembly technology, which combines a flip chip assembly and a wire-bonding assembly, is attractive in current semiconductor packaging. The hybrid assembly benefits the short interconnect paths between flip chip dies and a substrate, and utilizes wires to form tuning elements for flip chip dies. With the flip chip dies, the package space needed for wire bonding of dies is eliminated and thus the overall size of the package is reduced. In addition, the wires accompanied with the flip chip dies may be used to form jumpers, inductors, or other features that may be used to improve the performance of the flip chip dies.
In general, a flip chip die has a die body and multiple interconnect structures that are used to attach the flip chip die to the substrate. Each interconnect structure includes a solder and a pillar extending outward from the die body to the solder. In flip chip assembly, reflowing solders of the interconnect structures is one of the process steps used to connect each interconnect structure to a corresponding metal structure on a top surface of the substrate. Due to cost and performance considerations, the pillar of each interconnect structure and the corresponding metal structure are formed of copper. During the reflowing step, the solder of each interconnect structure will turn into a liquid-phase and flow along the most active surface. Since the pillar and the corresponding metal structure are formed from a common material—copper, there is a risk that the liquid phase solder will flow back to the flip chip die along the pillar and short circuits on the flip chip die. To address this issue, a surface finish is applied to the metal structure to increase its surface activity. One possible surface finish formed of gold will effectively pull the liquid-phase solder onto the metal structure and prevent the liquid-phase solder from flowing up the pillar back to the flip chip die.
In addition, in order to get a reliable wire bonding for a tuning wire, the substrate normally requires metal pads with high active surface finishes, which have high wire-bonding pulling strength. One possible surface finish formed of gold will effectively pull ends of the tuning wire onto the metal pads. However, the gold surface finish is expensive, which will raise the cost of the final product.
Accordingly, there remains a need for improved semiconductor package designs to reduce the possibility of the solders flowing back to the flip chip die and achieve the reliable wire bonding for the tuning wire. Further, there is also a need to keep the final product cost effective.